With the growing use of renewable power sources in distributed generation, grid connected converters (“GCCs”) are playing an increasingly important role as the interface between renewable energy sources and the utility grid. Filters, such as L-, LC-, or LCL filters, are used to attenuate the switching harmonics generated by GCCs. In many applications, LCL-filters are preferred in high-power GCC systems (e.g., systems with a power rating greater than 1 MW) due to their lower cost and superior harmonic attenuation capability as compared to L-filters.
However, a GCC incorporating an LCL-filter is a third-order system, which could cause instability problems and therefore make the control of the GCC difficult. Unlike a GCC incorporating an L-filter (also referred to as an “L-GCC” herein), very few vector control strategies for an LCL-based GCC (also referred to as an “LCL-GCC” herein) have been reported because of the difficulty to decouple the d- and q-axis control loops. One conventional vector control technique is to neglect the capacitor dynamics, thereby simplifying the vector control problem to that of a first-order L-GCC system. However, this results in imprecise description of the LCL-GCC system and potential oscillatory and/or unstable dynamic behavior if the LCL-filter or the GCC is not properly damped.
Conventional damping strategies for vector control of an LCL-GCC mainly fall into two categories: 1) Passive Damping (“PD”), and 2) Active Damping (“AD”). PD modifies the filter structure with the addition of passive elements such as resistors. AD modifies the controller parameters or the controller structure either by cutting the resonance peak and/or by adding a phase lead around the resonance frequency range. However, neither of these damping strategies solves the decoupling problem of an LCL-GCC system.